Porous materials, for example polymer foams, having pores in the size range of a few microns or significantly below and a high porosity of at least 70% are particularly good thermal insulators on the basis of theoretical considerations.
Such porous materials having a small average pore diameter can be, for example, in the form of organic xerogels. In the literature, the term xerogel is not used entirely uniformly. In general, a xerogel is considered to be a porous material which has been produced by a sol-gel process, with the liquid phase having been removed from the gel by drying below the critical temperature and below the critical pressure of the liquid phase (“subcritical conditions”). In contrast, the term aerogels generally refers to gels obtained when the removal of the liquid phase from the gel has been carried out under supercritical conditions.
In the sol-gel process, a sol based on a reactive organic gel precursor is first produced and the sol is then gelled by means of a crosslinking reaction to form a gel. To obtain a porous material, for example a xerogel, from the gel, the liquid has to be removed. This step will hereinafter be referred to as drying in the interests of simplicity.
WO-95/02009 discloses isocyanate-based xerogels which are particularly suitable for applications in the field of vacuum insulation. The publication also discloses a sol-gel-based process for producing the xerogels, in which known, inter alia aromatic, polyisocyanates and an unreactive solvent are used. As further compounds having active hydrogen atoms, use is made of aliphatic or aromatic polyamines or polyols. The examples disclosed in the publication comprise ones in which a polyisocyanate is reacted with diaminodiethyltoluene. The xerogels disclosed generally have average pore sizes in the region of 50 μm. In one example, mention is made of an average pore diameter of 10 μm.
WO-2008/138978 discloses xerogels which comprise from 30 to 90% by weight of at least one polyfunctional isocyanate and from 10 to 70% by weight of at least one polyfunctional aromatic amine and have a volume average pore diameter of not more than 5 microns.
The unpublished EP-A 09178783.8 describes porous materials based on polyfunctional isocyanates and polyfunctional aromatic amines, where the amine component comprises polyfunctional substituted aromatic amines. The porous materials described are produced by reacting isocyanates with the desired amount of amine in a solvent which is inert toward the isocyanates. The formation of urea linkages occurs exclusively by reaction of the isocyanate groups with the amino groups used.
However, the materials properties, in particular the mechanical stability and/or the compressive strength and also the thermal conductivity, of the known porous materials based on polyurea are not satisfactory for all applications. In addition, the formulations on which the materials are based display shrinkage, with reduction of the porosity and an increase in the density, on drying. Furthermore, the gelling time, i.e. the time required for gelling of the starting compounds, is often too long.
A particular problem associated with the formulations based on isocyanates and amines which are known from the prior art are mixing defects. Mixing defects occur as a result of the high reaction rate between isocyanates and amino groups, since the gelling reaction has already proceeded a long way before complete mixing. Mixing defects lead to porous materials having heterogeneous and unsatisfactory materials properties. A concept for reducing the phenomenon of mixing defects is thus generally desirable.